1. Field of the Invention
The invention is directed to a process for fracturing a tight subsurface formation, and more particularly to a two-step process for fracturing an oil shale formation for production of shale oil.
2. Description of the Prior Art
Historically, pathways in hydrocarbon formations have been created using high explosives, hydraulic fracturing techniques, and acid treatments. Research has shown that the pressure pulse created by high explosives enlarges the well bore by crushing and compacting the surrounding rock in the formation. The enlarged well bore is left with a zone of residual compressive stress and compacted rock which can actually reduce permeability near the well bore. Extensive caving also occurs in the well bore leaving debris that may require days or even weeks to clean up. Hydraulic fracturing is highly effective but is well known to create fractures that can break out of a producing formation and into nearby aquifers.
A more recently available alternative used for fracturing a tight formation surrounding a well bore is the use of high pressure gas pulses, sometimes referred to as high energy gas fracturing. This involves activating a solid propellant, often referred to as a low explosive, to generate high-pressure gas pulses that are strong enough to create multiple fractures in the adjacent formation radiating 10 to 100 feet from the well bore, but not so strong as to pulverize and compact the rock such as is the case with high explosives. It is sometimes explained that these solid propellants do not detonate supersonically, but deflagrate at subsonic velocities. Several advantages of high-pressure gas pulse technology are that cavings are minimal, the integrity of the well bore is maintained, and clean up is nominal. The nature of the forces produced by gas pulses also has the salutary effect of creating fractures having minimal vertical propagation thereby lessening the chances of breaking into adjacent aquifers.
Ammonium-nitrate fuel oil (ANFO) is one of a class of high explosive compositions which includes an oxidizing agent and a liquid hydrocarbon component. Ammonium-nitrate is by far the preferred oxidizing agent and #2 fuel oil is usually the liquid hydrocarbon of choice. ANFO may be modified to reduce the shock energy and increase the heave energy of the explosion. ANFO so modified, commonly known as a low shock energy explosive, has been used in quarrying operations in rock blasting situations, but has never been used for fracturing oil shale formations.
It is critically important to avoid contamination of aquifers located in proximity to the targeted portion of the formation. The vertical fracturing which results from hydraulic fracturing techniques can intersect natural vertical fractures in the formation creating pathways to aquifers usually located in horizontal strata above or below a target formation.